Non-Dispersive Infrared (NDIR) gas sensors have been in existence for over six decades—ever since the technology for the fabrication of spectral thin film narrow band pass filters was declassified by the United States government in the late 1940's. Although numerous gas detection methodologies have been developed in parallel over the same time span, the most notable ones include electrochemical fuel cells, Figaro or tin oxide (SnO2) sensors, metal oxide semiconductor (MOS) sensors, catalytic (platinum bead) sensors, photo-ionization detectors (PID), flame-ionization detectors (FID), thermal conductivity sensors etc., all of which are commonly referred to as “interactive” types of gas detectors. As interactive types of gas sensors, almost every such detector without exception suffers from long-term drifts and non-specificity problems.
NDIR gas sensors, on the other hand, have long been considered as one of the best methods for gas measurement since the 1950's. Comparatively speaking, in addition to being highly specific, NDIR gas analyzers are also very sensitive, fast responding, relatively stable over time, rugged, reliable and easy to maintain. A superior gas measurement technique, which only became fully developed within the past two decades, is the tunable diode laser absorption spectroscopy (TDLAS) methodology. This technique uses the output of a laser as a tunable coherent radiation source. TDLAS gas sensors in general possess better gas detection sensitivity, and in some cases, can be more compact and reliable than the corresponding NDIR counterparts. However, due to available laser wavelength restrictions, they have to depend upon the much weaker higher harmonics of the fundamental absorption bands of gases that they detect. Consequently, TDLAS gas sensors routinely require many times the path lengths required for their NDIR counterparts for attaining comparable detection sensitivities and they are, at least at the present time, a lot more expensive because of the limited availability of lasers (particularly of the Quantum Cascade types).
The over six decades of incessant technology development for NDIR gas sensors began with the advent of the “Single Beam Methodology” exemplified by the Beckman LB-1 medical CO2 sensor circa 1951-1955 which suffered from extremely poor output drift problems over temperature and time. The emergence of a variety of “Double Beam Methodology” took place during the period between the early 1960's and the mid-1970's, most notably represented by the development of Hewlett-Packard's Model 47210A Capnometer™ which was also a medical CO2 sensor for monitoring the end-tidal CO2 levels of cardiac and respiratory patients in ICU's (see Jaffe, M B “Infrared measurement of Carbon Dioxide in the Human Breath: “Breathe-Through Devices from Tyndall to the Present Day “Technology, Computing and Simulation 2008; 107: 890-904). In the Capnometer, a novel double beam technique known as “Negative Filtering” was advanced in the creation of a two-beam ratio signal processing scheme. This “Double Beam Methodology” was later improved by Burch et al. in U.S. Pat. No. 3,793,525 (1974) and by Blau Jr. in U.S. Pat. No. 3,811,776 (1974) in manners very similar to the “Negative Filtering” concept first introduced in the Hewlett-Packard's Capnometer.
As the fabrication technology for narrow band pass interference filters gradually advanced towards the end of the 1970's, gas cells that were used earlier for implementing NDIR Double Beam methodologies were quickly replaced by these filters. A simpler and less costly Double Beam methodology would involve a beam of infrared energy emanating from an infrared source and passing through a sample chamber containing an unknown amount of the gas whose concentration is to be determined. Before reaching an infrared detector, the beam was passed through two narrow band-pass filters which were mounted on a rotating wheel. One of the two filters would pass only radiation at the characteristic absorption wavelength of the gas to be detected. The other filter was used as a reference filter at a wavelength close to, but not overlapping, that of the first filter. This type of NDIR gas analyzer implementing the Double Beam methodology required generation of some type of synchronizing signal in order to coordinate operation of the signal processing circuit with rotation of the filter wheel.
The period from the early 1980's to the early 1990's witnessed a rapid growth of diverse applications in the use of NDIR gas sensors. Besides the medical and HVACR industries which have been the mainstay for the need of NDIR gas sensors throughout the 1970's, other industries, such as mining, oil and gas production, diverse manufacturing, pharmaceutical etc., where safety and efficiency are invariably of the utmost importance, started to demand more and more NDIR gas sensors. Their demand did not focus just on the general availability in numbers and their detection of diverse gases, but also in the size and ruggedness for these sensors. Throughout the 1970's NDIR gas sensors, especially those that implemented the Double Beam methodology of gas measurement, were bulky, relatively heavy and included moving parts such as mechanical light choppers. Beginning in the mid-1980's, researchers and developers of NDIR gas sensors concentrated on new sensor designs that were compact, light-weight and possessed no mechanical moving parts nor focusing optics. Such designs can be found in U.S. Pat. No. 5,026,992 entitled “Spectral Ratioing Technique for NDIR Gas Analysis Using a Differential Temperature Source” by Wong (Jun. 25, 1991), in U.S. Pat. No. 5,340,986 entitled “Diffusion-Type Gas Sample Chamber” by Wong (Aug. 23, 1994) and in U.S. Pat. No. 5,341,214 entitled “NDIR Gas Analysis Using Spectral Ratioing Technique” by Wong (Sep. 23, 1994).
One of the most noteworthy new designs for NDIR gas sensors was disclosed in U.S. Pat. No. 5,163,332 entitled “Gas Sample Chamber” by Wong (Nov. 17, 1992). A diffusion-type gas sample chamber for use in a gas sensor consists of an elongated hollow tube having an inwardly-facing specularly-reflective surface that permits the tube to function also as a light pipe for transmitting radiation from a source to a detector through the sample gas. A number of filtering apertures in the wall of the otherwise non-porous hollow tube permit the sample gas to enter and exit freely under ambient pressure. This invention for a simplified diffusion-type gas sample chamber provides a novel approach for reducing the complexity of NDIR gas measurement systems by eliminating the need for expensive optics, mechanical choppers and a pump for pulling or pushing the gas into the sample chamber. In addition, the sample chamber of this invention provides a long effective path length which increases the gas detection sensitivity.
From the mid-1990's onwards as the need and use of NDIR gas sensor for detecting all manners of gases in diverse industries continued to grow unabated and the research and development efforts concentrated in new designs that would not only improve their performance characteristics but also reduce the overall sensor cost with the use of multi-channel gas sensors. Many manufacturers took advantage of the so-called waveguide hollow tube sample chamber concept (see U.S. Pat. No. 5,163,332 cited above) and were able to introduce NDIR gas sensors that were significantly lower in unit price. New sensor designs that continued to take advantage of achievements discussed above can be found in U.S. Pat. No. 5,222,389 entitled “Multi-Channel Gas Sample Chamber” by Wong (Jun. 29, 1993) and in U.S. Pat. No. 5,502,308 entitled “Diffusion-Type Gas Sample Chamber” by Wong (Mar. 26, 1996).
In the first of these two disclosed new designs, several detectors equipped with different narrow band-pass interference filters as windows are mounted at the detector end of the so-called waveguide hollow tube sample chamber (see U.S. Pat. No. 5,163,332). By virtue of the fact that the waveguide sample chamber serves as a light pipe to conduct radiation via multiple reflections inside the highly reflective wall, the entire sample chamber is uniformly illuminated with radiation at a slowly decreasing intensity towards the detector end. Thus, at the detector end each of the several mounted detectors essentially receives the same radiation intensity from the common infrared source. Furthermore, each of the common source-detector pair has approximately the same path-length. Thus, if each of the several mounted detectors carries a different narrow band-pass filter that passes radiation which is absorbed by a particular gas present in the gas sample chamber, this new design essentially functions as a compact and low-cost multi-channel NDIR gas sensor.
In the second of the above disclosed new designs, a gas filter cell, inserted to the source/detector end of the waveguide sample chamber (see U.S. Pat. No. 5,341,214), is used to significantly reduce the influence of an interference gas present in the sample chamber. Since the radiation source in this new design is mounted at the same end of the sample chamber as the detector, radiation emitted by the source is reflected from the other end of the sample chamber back to the detector after passing through the gas filter cell twice. The gas filter gas is filled with the interfering gas. In passing twice through the gas filter cell, the radiation generated by the source is greatly attenuated at wavelengths corresponding to the absorption bands of the interfering gas. Since interference occurs only at wavelengths where the absorption bands of the interfering gas overlap the absorption bands of the gas to be detected, the great attenuation of the radiation at such wavelengths by the gas filter cell substantially eliminates the possibility of interference.
New NDIR gas sensor designs continued to be introduced well into the 2,000's aimed at further improving sensor performance and reducing unit production cost for NDIR gas sensors. In U.S. Pat. No. 7,259,374, entitled “Method for Detecting a Gas species Using a Super Tube Waveguide” by Wong (Aug. 21, 2007), the concept of using a hollow tube with an inwardly-facing specularly-reflecting surface as a functionally efficient sample chamber was extended from a one-dimensional straight tube to a multi-bend waveguide collectively greater than 180 degrees in three dimensions. By so doing, the effective sample chamber path length of NDIR gas sensors can be elongated by more than an order of magnitude to 60″ or longer. This new design can allow NDIR gas sensors to detect gas concentrations down to 1 ppm or less.
Another noteworthy new design for NDIR gas sensors was recently disclosed in U.S. Pat. No. 7,358,489 entitled “Ultra Low Cost NDIR Gas Sensors” by Wong (Apr. 15, 2008). In this new design, the concentration of a gas species is detected by using a single beam NDIR gas sensor in which an infrared source element is driven at two different temperatures. A feedback loop is designed to sense an operational voltage of the source. A differential gain amplifier is further designed to create a high cycle amplified output and a low cycle amplified output during respectively a high and low cycle pulsing of the source. Meanwhile a controller is used to synchronize the source driver so that a signal processor can determine the gas concentration through the use of the high and low cycle amplified outputs. This new methodology could further afford the use of a non-genuine blackbody source such as an incandescent miniature light bulb in order to minimize the unit production cost for this sensor.
Despite the over six decades of incessant technology development for NDIR gas sensors, there are still two important sensor performance deficiencies that have yet to be overcome. The first one is sensor output stability over time and the second one is sensor output inaccuracies due to other gases present with the gas to be measured because of interferences caused by the overlapping of their absorption bands. One of the most important sensor performance characteristics is indeed the sensor output stability over time without the need for periodic re-calibration. It is because of the fact that without an output stable CO2 controller, for example, the implementation and practice of Demand Controlled Ventilation (DCV) strategy in office and commercial buildings to save energy would be very awkwardly inconvenienced. However, until the middle of the 1990's, no design was yet in sight to remedy this serious deficiency for NDIR gas sensors. It appeared that the only interim solution for solving this problem was through use of sensor output correction software.
Such a sensor was disclosed in U.S. Pat. No. 5,347,474 (1994) entitled “Self-Calibration of an NDIR Gas Sensor” by Wong. This methodology is based upon the concept that some cyclic variables include within each cycle a value that can be extrinsically determined. In such a case, the sensed value may differ from the known value by an amount that is a combination of long-term drift of the sensor and random measuring error. The drift component can however be evaluated and eliminated by devising a specific method as follows. Once each cycle, for a number of cycles, the sensor measures the variable at a time when its value should equal the extrinsically-known value. The differences are plotted versus time, and a best-fitting straight line is determined, which indicates the drift. Throughout the next cycle as the variable is continuously sensed, the drift determined from the best-fitting straight line is continuously applied in small quantities in order to correct the sensed value.
However, this application builds upon U.S. patent application Ser. No. 12/759,603, wherein a new NDIR gas sensing methodology is advanced which renders to first order the output of an NDIR gas sensor designed using this methodology virtually drift-free in time without the need for any sensor output correction software or periodic re-calibration. Thus, it appears that the first and important sensor performance deficiency has now been overcome.
But the second and important sensor performance deficiency, namely cross interferences among gases in a sample when the concentration level of only one of them is of interest to be detected or measured, remains unresolved. Among the most well known examples of such instances are the overlapping of the 2.70μ absorption bands of CO2 and H2O. For this reason, the 4.2μ absorption band of CO2 gas is preferred for the detection of CO2 instead of the 2.70μ absorption band. Since H2O is always present in the ambience at various unpredictable levels, its interference on CO2 measurement in the ambience would cause unacceptable inaccuracies. Similarly, the 2.70μ absorption band for H2O cannot be used to detect this gas due to the interference by CO2 and its variable and unpredictable amount normally found in ambient air. This application specifically addresses this performance deficiency and provides a cure for it.